Existing developer rollers which this invention improves upon are typically semiconductive rubber with a thin outer, insulation layer of polyurethane. Such developer rollers have a conductive center shaft to which an electrical potential is applied which is approximately one half of the potential to which non-imaged areas of a photoconductive surface are charged, with the imaged areas being substantially at zero volts. Toner powder is separately charged, applied to the developer roller, and regulated in thickness and charge by a electrically charged doctor blade as the developer roller is turned. The developer roller then makes nip contact with the photoconductive surface, which typically is a roller or belt moving with the developer roller.
The parameters of the foregoing system can be selected to develop an optimum image on the photoconductor at given ambient characteristics. However, at start-up and at extreme environments, such parameters are ineffective since the machine is operating out of the range assumed in their selection. Specifically, when the system is relatively cool, a rubber roller will be significantly less conductive and the reduced conductivity will result in lines in the printing being narrower. Increasing the potential to the developer roller would correct the start-up images but cause later images to be wide and dark. This invention adjusts the conductivity of the rubber of the developer roller to overcome this problem.
U.S. Pat. No. 4,710,015 to Takeda et al teaches such a developer roller and system, but the additives to its developer roller rubber; copper, nickel, and carbon black; are not disclosed as being for the purpose of curing start-up and similar problems, and such additives are believed to undesirably change the mechanics of the roller and to make the roller more conductive than is desirable to cure such start-up problems. U.S. Pat. No,. 4,100,884 to Mochizuki et al, particularly the FIG. 3 embodiment, is of interest for its showing of a rubber roller.
The preferred embodiment of this invention is a change in the developer roller formulation to add a quaternary ammonium material and is otherwise identical with the prior printer sold by the assignee of the invention with the name Laser Printer and the number designation 4019.